Silver chlorobromide photographic emulsions have better developability than silver iodobromide emulsions and are conventionally used in printing photosensitive materials or color paper. Since they have low sensitivity, silver chlorobromide emulsions must be used in large grains to achieve higher sensitivity, but increased grain size impairs their graininess and developability.
Chemical sensitization is commonly used to increase the sensitivity of silver halide photographic emulsions, and conventional methods of chemical sensitization include sulfur sensitization, gold sensitization and reduction sensitization which are generally used either alone or in combination. It is known that the combination of sulfur sensitization and gold sensitization achieves a higher sensitivity than sulfur sensitization alone, but this combination is not practical for the purpose of sensitizing silver halide photographic emulsions substantially made of silver chlorobromide since excess fog results. Therefore, it has long been desired to develop a method of sulfur sensitization that can effectively sensitize silver chlorobromide photographic emulsions without the help of gold sensitization.